Pulsed optical fiber amplifiers have recently gained considerable interest in applications, such as material processing, printing, and light detection and ranging because of their capability to reliably produce nanosecond pulses with high average and peak power, and good beam quality. Recent developments in double clad ytterbium doped large mode area (“LMA”) optical fiber amplifiers have led to record combination of average and peak output powers at 1064 nm. Due to the very high peak intensities of the amplified pulses in such optical fiber amplifiers, nonlinear effects (e.g., stimulated Brillouin scattering (“SBS”) and stimulated Raman scattering (“SRS”) can limit the extracted pulse energy.
One prior art technique for reducing nonlinear effects is to increase the effective mode area of the optical fiber by decreasing the core numerical aperture and increasing the core diameter thereof. Recently, photonic crystal or “holey” fibers have been used to increase the mode area of an optical fiber. However, the viability of increasing the effective mode area of single mode LMA optical fibers can be limited due to the increased sensitivity to bend-losses when the effective mode area is increased, which can limit the manufacturability or practical utility of such optical fibers and can cause even the lowest order transverse mode to leak out.
A variety of prior art techniques are available that can assist with maintaining single-transverse mode output in multimode optical fibers. Such techniques include selective bend-losses in coiled optical fibers, control of the seed conditions, design of radial index and dopant profiles, and use of helical-core optical fibers. However, the desire for larger output powers leads to operation of these pulsed optical fiber amplifiers at the maximum peak power for the given optical fiber core dimensions so that nonlinear effects can remain an issue. Limiting effects on the output power, spectral bandwidth, or temporal profile depend on the temporal regime. SBS is one of the most limiting factors for long pulses (>˜10 nanosecond) and self-phase modulation induces very large distortion of the temporal profile and spectra for short pulses (<1 nanosecond). Accordingly, one prior art approach shows that a pulse duration of approximately 1 nanosecond is suitable for minimizing the above-mentioned nonlinear effects in high energy pulsed optical fiber amplifiers. When a pulse duration of 1 nanosecond is used, SRS generally becomes the most limiting effect.
Another nonlinear effect in pulsed optical fiber amplifiers is four-wave mixing (FWM), which can lead to a relatively broad output spectrum in a pulsed optical fiber amplifier. FWM in an optical fiber amplifier results when two waves from the amplified beam combine to produce so-called “signal” and “idler” waves in which the signal wave has a wavelength greater than the wavelength of the amplified wave and the idler wave has a wavelength less than the wavelength of the amplified wave. FWM is efficient when the four interacting fields remain phase-matched as they propagate along the length of the optical fiber. However, in single-mode isotropic optical fibers with normal dispersion, which is usually the case of optical fiber amplifiers, the extent of phase-matching is very limited so that FWM remains a very weak effect.
As will be described more fully below, the inventors have found that FWM can significantly limit the output power in pulsed optical fiber amplifiers. Accordingly, there is still a need in the art for increasing the output power in optical amplifiers such as pulsed optical fiber amplifiers while taking into account the limiting effect of FWM.